Tag Archives: Weaver Creek

Well, it’s been a week and a half since the snow came down, and there are plenty of shady areas where it’s still on the ground. We started out on the east side of the map in Deadland Hollow, even though it was north-facing, because we knew we could get to it fairly easily. If anything, the snowy areas may even have been a little more treacherous this week, because it’s thawed and frozen so many times that it’s more like solid ice now. After that we went all the way over to the west side of the map and got a few points in a small drainage south of Weaver Creek.

On Tuesday we walked from the lake to the top of Dave Creek in an undeveloped part of Fairfield Bay. Saw some really good worm burrows in a thin- to medium-bedded sandstone near where the creek reaches the lake. Some of these burrows crossed bedding planes, so the rate of sedimentation must have been fairly rapid during deposition of this unit.

Above that, we were in fairly continuous outcrops of calcareous sandstone, including some beds of “zebra rock” (see previous two posts). We can only surmise that we are again in the Witts Springs Formation, which is interesting in that it’s still at the surface south of that Weaver Creek/Middle Fork lineation. This probably means that if the lineation formed due to a fault at the surface, there is minimal offset. More likely, it indicates the lineation formed along a monocline at the surface perhaps indicating a fault at depth in the basement rock. Anyway, we had several hundred feet of calcareous sandstone along the creek, some of which exhibited large scale cross-bedding.

We’re still working on that Bloyd/Witts Springs contact, but probably did cross it somewhere in that upper end. There may even be some Atoka up there, but if so, it will be in sparse exposures at the very highest elevations. If the Bloyd proves to be several hundred feet thick here as it has been on other quads to the west, the only Atoka may be on the southern third of the Fairfield Bay quad south of the southwest/northeast lineation that goes through the upper end of Greers Ferry Lake. This is almost undoubtedly a fault at the surface based on the steep dips we’ve been seeing on the north edge of the lake.

On Wednesday, we started at the lake again and went up a small drainage on the eastern edge of the map. Had a great view of Sugar Loaf Mountain when we started that morning.

Well, I hate to say it, but this will be the final blog of this field season. I have a major test and a GSA field trip to prepare for this spring, so I need to devote all my time to that right now. It’s been fun to write it, and I hope I’ve given you a better idea of what range of effort goes into making a “simple” geologic map. We’ll keep going out until mid-April, then we’ll have about 10 weeks in the office to draw and digitize the two maps, add descriptions of the rock units, a cross section, stratigraphic column, joint diagram, and correlation of map units. If all goes according to plan, we’ll turn the finished maps into the USGS on June 30 to fulfill our grant requirements. The Shirley and the Fairfield Bay quads should be up on our website as a .pdf by mid-July. By that time, we’ll be back in the field battling ticks and snakes next season–probably on the Parma and Greers Ferry quads. Who knows, maybe I’ll start blogging again! Until then, see you on the outcrop!

Well, we did have a winter weather event last Friday that was still on the ground in the field area the following Tuesday. Roads were fairly clear to Clinton, but between Clinton and Shirley it was still mostly patches of snow and ice. North of Shirley, the roads were pretty much all covered with snow and ice. Even “Goldy” (our Jeep) got stuck briefly when Danny decided to stop and check his map almost at the top of a hill.

Tuesday we managed to get to a drainage south of the Middle Fork that had fairly low relief, so were able to climb down the side and follow it on down. There were some very large footprints in the snow along the valley floor that may have been a feral pig, but don’t know for sure. It’s amazing how the critters always choose the easiest route. Their trails are usually pretty good for people too, though they don’t often care about avoiding briar patches. The snow was pretty crunchy, so fairly good traction. Having snow on the ground rather evens out the terrain in an odd way, though you have to be poised to catch yourself with every step. Had some massive sandstone units, but they were blocky and non-calcareous, so we’re in something different than on the north side of the river.

The next day we managed to make our way north of Shirley to a couple drainages at the head of Indian Creek that we had skipped when we did the lower end. As it happened it was a pretty good choice because the relief was rather low and the bottom was fairly wide. Also, it was south-facing which helped to melt the crusty snow and maybe keep us a little warmer. We saw mostly massive calcareous sandstone units of the Witts Springs though may have gotten into the Bloyd in the higher elevations. At the end of the day we entered an area that still showed signs of damage from the Jan. 2009 ice storm that coated most of northwest Arkansas with a thick layer of ice and downed many trees–many by the roots. Witnessed a beautiful sunset on the way out though. We were near 1400 feet which is about the highest elevation on the quad. The trees there were still coated in ice from last Friday’s storm.

Thursday we tried another drainage near the one we hiked on Tuesday. This one was north-facing and proved to be too steep and narrow a descent to do with so much snow on the ground. The snow had developed a thick icy crust from thawing and refreezing, and we had to break through it to get traction. I managed to get up and around on the side of a particularly narrow spot in the creek bed, and was waiting for Danny to follow when I saw him retreating back upstream. I later found out that he couldn’t find purchase on the icy banks, so decided to return the way we had come. Since I had gotten farther downstream by breaking through the crusty snow and climbing down the steep side, I had no choice but to climb back up that way. I found that going on up was less hazardous than going back down toward the creek, so I continued to climb up the side and by the time I drew even with Danny was probably 100 feet higher. I could barely see him down there, but managed to get a photo of him climbing up the drainage. We’ll have to try that one again when the snow is gone!

After that, we got a few points on Weaver Creek were it leaves the western edge of the map, and headed back to Little Rock. Hope this clears up by next week! Until then, see you on the ice-covered outcrop!

This week we finished up a few odds and ends on the Shirley quad. We needed to get to a few suspected outcrops along the north side of the Middle Fork just east of Shirley. As we were looking for a way to access them, we stumbled upon the Sid Burgess Historic M&NA Trail which starts in downtown Shirley and ends up about a mile distant at the historic Cottrell-Wilson Cemetery. As luck would have it, this trail happened to access the very areas we needed to see. If you’re ever in Shirley, it’s definitely worth checking out!

We saw mostly thin-bedded sandstone and shale units of the same variety as on the south side of the Middle Fork and Weaver Creek upstream. There are a few low dips toward the lineation, but nothing indicating a major structure. I’m thinking this may all be the unit above the Witts Springs (Bloyd Formation) brought down to the southeast by a monocline. The trouble is, we don’t really know what the Bloyd/Witts Springs contact looks like in this area yet. That’s something we still need to work out.

Tuesday was wet again, but we set out to finish up the southernmost branch of Lost Creek anyway. Seems to be mostly Witts Springs in there with some Cane Hill at the bottom of the valley. We saw some great examples of soft-sediment deformation in some of the silty units on the way down. Soft sediment deformation occurs during sedimentation when the rapid loading of usually more dense, overlying sediments causes the less dense, buried deposits beneath them to become partially liquefied, which forms various types of disruptions in the original bedding. This can take the form of simple reorientation of the bedding as we have here, to more complex convolute bedding and flame structures. I took a photo later in the week of a good flame structure in the Bloyd Formation. Notice where the shale has been squeezed up between the thick, contorted beds of sandstone.

Several massive calcareous sandstone units in the Witts Springs again illustrated the dramatic difference between outcrops weathered with and without the influence of groundwater. Notice how rotten the outcrop of massive sandstone in the photo below left appears. Also note the green color. There is a layer of moss and lichen growing over almost the entire rock surface, made possible by its relative saturation by groundwater. These organisms help accelerate the weathering of the rock, and there are places where you can actually see clumps of moss peeling off the surface along with a layer of sand. This type of chemical weathering is known as chelation and results in the effective removal of the residual iron cement still holding the rock together after the calcite cement has been dissolved by groundwater. The photo below right shows how “dry weathering” of a boulder of the same material can result in well-defined liesegang bands. Highly concentrated iron has cemented these bands within the massive sandstone, and without the influence of groundwater, they are preferentially resistant to weathering, leaving them in bold relief.

On Friday, we looked at some of the last steep areas we haven’t vistited north of the Middle Fork east of Shirley. Definitely still have Witts Springs right down to the river there, but there is also a thin- to very thick-bedded unit above it that is probably in the Bloyd. We saw a fairly recent landslide above the river composed of material from that upper unit. There was also a good cut and fill channel bed exposed in that unit as well.

It was warm enough for the critters to be out again this week. Just when I thought it was safe to put my foot down anywhere I pleased, I nearly stepped on a moccasin. That’s him slinking back in his hole. We also saw a western slimy salamander (plethodon albagula?) under some storm debris, which was subsequently replaced.

Hopefully the warm weather holds out, but the forecast says the bottom may drop out on Friday. We’ll see!

Well, another great week to look at rocks! We explored about a five mile stretch of Big Branch, definitely the biggest drainage left unexplored on the Shirley quad at this point. Quite a bit of Cane Hill in the bottom, then several hundred feet of Witts Springs above. The rocks near its confluence with Weaver Creek are dipping strongly southeast, and the Cane Hill actually dives into the subsurface there.

Still haven’t decided if the big structure in Weaver Creek valley is a fault or just a really big monocline, but we’re leaning toward monocline right now because we still haven’t seen a real break in the rocks. Of course, faults typically become covered because the fractured rock is preferentially eroded. It just seems less and less likely that there is a fault there with each drainage we do that should cut across it.

Monday we walked in the lower end and got several strong SE dips in the Cane Hill. Then we discovered an outstanding outcrop of basal Witts Springs sandstone, that we thought was a very large boulder at first because of the advanced state of the sort of “dry weathering” that usually affects the massive Witts Springs boulders after they become separated from groundwater, usually along joints, as they slide downslope. This includes well-developed honeycomb taphoni, well-defined liesegang banding, and case-hardening of the surface. In the bluff face, solutionally-enlarged joints can form fracture caverns, and spalling near the base can form bluff shelters. All of this can happen under the influence of groundwater of course, but that kind of saturation usually leads to a punky or rotten texture in the rock, and forms very steep, covered topography. The really spectacular outcrops occur when lack of groundwater slows down the weathering to a grain-by-grain process. This is what I call “dry weathering”. After walking up both sides and along the top, we concluded that it was indeed part of a continuous outcrop that was probably protected from groundwater penetration by its joint system. I dubbed it “Castle Rock” because of its many turrets and towers.

Speaking of weathering, on Tuesday we saw a classic example of preferential weathering along beds of sandy limestone interbedded with limey sandstone. When these beds are freshly exposed, they form light and dark bands within a smooth face of massive sandstone. We refer to this informally as “zebra rock”. The light bands are more limey, the dark bands less. As weathering progresses, chemical weathering breaks down the more limey areas at an accelerated rate simply because there is more reactive material in that rock than in the sandier beds around it. When weathered, these areas form long horizontal hollows or pits in the massive sandstone.

We informally refer to this weathering pattern as “Prairie Grove weathering” after a Member of the Hale Formation in northwest Arkansas that most typically exemplifies this trait. The base of the Witts Springs Formation is an equivalent unit to the Prairie Grove Member, and often massive sandstones within the Witts Springs will display this type of weathering as well. Though not definitive, this characteristic can be used to help us determine if a given outcrop is within the Witts Springs.

As we made are way up the Big Branch, we ran into our old nemesis: the February 5, 2008 tornado track. This is the one that was on the ground for 122 miles in Arkansas and killed 14 people. We’ve crossed it’s track on several maps, and it never ceases to amaze how destructive it was.

Another good week in the field! Continued fairly dry fall weather means we’re still in the big drainages this week. Finished up in Weaver Creek on Monday. Still have the channel-bedded mostly shaly unit in the valley and the thin- to very thick-bedded sandstone unit on the southeast side of the valley. Got to see the thickness of the alluvial cover in the high wall of a shale pit in the middle of the valley. The owner states that the pit is 60 feet deep and is shaly to the bottom. That’s a lot of shale! The sandstone above it is at least that thick and probably more like 100 feet. Still seeing channel beds within the sandstone unit as well. A ribbon snake crossed our path heading toward the water.

The next day, we tried to cross the Middle Fork as we have done in past weeks, but there is already too much water in it. There are some deep hollows on the east side that we needed to get to, but after fording the river proved impracticable, we resigned to going north on highway 9 and dropping off the top edge of the valley. At least we started from the lowest saddle. It was a fairly easy descent with a few washed out 4-wheeler trails helping out. We knew right away we were in Witts Springs when we reached the bottom, but were surprised by how low the contact was when we reached the Cane Hill. When we reached the river valley, we decided to follow a slough upstream to the next drainage on the east side. This looked like the best route on the map, but was actually a thick canebrake with very few places to cross the slough. We had a hard time getting in the upper end of this drainage a few weeks ago, and the lower end proved no different. We were rewarded by finding a lot of deformation bands along non-vertical joints in the Witts Springs, and a drop of about a hundred feet in the contact with the Cane Hill between the drainages, which are only separated by about a mile. At best may be a monocline in there. On the way out we nearly ran over what I hope is the last moccasin of the season sunning on a very steep hillside above the drainage. We gave him as wide a berth as we could and continued back to the river bottom. This time we tried to avoid the slough and stayed closer to the river where the switch cane was thinner. We backtracked up the hollow we came down earlier, then climbed back up the side to the highway. So to summarize, river unfordable so drove around, hiked down 420 feet, bushwhacked a mile through canebrake, hiked up 320 feet, saw deformation bands, back down 320 feet, avoided snake, hiked a mile through woods, climbed back up 420 feet. What a day!

On Wednesday we started on the west side of the river downstream of where we were the day before, and walked up one of two remaining large drainages before the river turns northeast at Shirley to follow the lineament. We were rather surprised to find Imo near the mouth of the hollow. This would make it the southernmost exposure of Imo mapped to date. Saw good exposures of Cane Hill, which is mostly sandy through here, on the way to a classic example of the basal Witts Springs sandstone complete with exfoliation weathering.

Once we got out of there, it was back to Little Rock and back to reality.

Oh, and I almost forgot. I found this graph on the USGS website and thought you might find it interesting. You can see that flood event on the last day of May this year that we are still seeing evidence of. Apparently it was right up there with some of the biggest recorded events on the Middle Fork in that area.

Looks like we’ll be in the office next week, so next installment in two. Until then, I’ll see you on the outcrop!

It was a rather wet week in the field this week. Starting to see a little fall color as well, but still mostly green. We worked in Weaver Creek which runs along that southwest/northeast lineation on the southern part of the Shirley quad. It appears that the wide valley there and in the Middle Fork to the northeast is developed in a very thick shale sequence that is all dipping southeast, probably southeast of the major fault causing the lineation. The valley can widen significantly in the shale, but once it reaches the 200+ feet thick, thin- to massive-bedded sandstone unit on the southeast side of the valley, it can’t go much farther. Essentially the lineation is caused by the stream eroding upsection across the valley through the shale unit, until it reaches the contact with the thick sandstone unit above. The lineation is therefore along the strike of (perpendicular to) the southeast dipping rock in this area. One big question is what happens to the fault on the east side of the map where the lineation stops. May have to wait to find out next year when we map the Parma quad.

Started out on Buffalo Hump, which I think may be a reference to a series of rounded “humps” along the end of Weaver Creek where the sandstone bluff is deeply incised by four drainages.

Saw several sets of non-vertical joints crossing each other in one of those drainages. This usually indicates that there is a structure close-by, but we didn’t see anything unusual.

The next two days were rainy, so we stayed close to the highway so we could bale out in the event of a deluge. The shaly unit was well exposed in the valley and displayed abundant soft-sediment deformation and load casts.

It was difficult to ascertain the true dip of the rocks because this unit is a stack of very broad channels, so the beds dip consistently for a long way, then abruptly turn about 90 degrees and continue on like that for a ways, then switch back. The exposed edges of these channels typically dip sharply and are sandier. Wish we had more time to study this unit to determine paleocurrent direction, facies, and depositional environment, but as usual, we’re just passing through.

The channel-bedding continues into the sandy unit above as well. The big question is what formation these units are in. Now that we’ve worked our way south of the fault, we’re in a totally different section than we were in north of it. Still haven’t seen anything that looks too familiar. May have to pull well logs and work our way up from the subsurface to get started.